An infrared energy detector is a device which produces an output signal which is a function of the amount of infrared energy that is impinging on an active region of the detector. There are two types of infrared detectors, photon detectors and thermal detectors.
Photon detectors function based upon the number of photons that are impinging on a transducer region of the detector. Photon detectors for infrared radiation are typically made of small band gap (about 0.1-0.2 eV) semiconductors such as HgCdTe and these detectors operate as photodiodes or photocapacitors by photon absorption to produce electron-hole pairs. Photon detectors are relatively sensitive and have a high response speed compared to thermal detectors. However, the small band gap is only about 4KT at room temperature and as a result dark current swamps any detectable signals. Photon detectors operate well only at low temperatures and therefore require refrigeration by liquid nitrogen (LN) to provide sensitive detection. Moreover, for wavelength greater than 20 .mu.m, there is no satisfactory cooled detector technology above liquid helium (LHe) temperatures.
Thermal detectors function based upon a change in the temperature of the transducer region of the detector due to absorption of the infrared radiation. Thermal detectors provide an output signal that is proportional to the temperature of the transducer region. Since radiation absorption usually occurs over a wide range of wavelengths, thermal detectors are typically responsive over a wide range of wavelengths.
A bolometer is a thermal detector for infrared detection having a transducer region made of a material which has its resistivity change as the temperature of the material increases in response to the infrared energy impinging on, and absorbed by, the material Thus, in response to the change of resistance, by connecting the material to a constant voltage supply, the electrical current through the material will vary in accordance with the infrared energy sensed by the material or by connecting the material to a constant current supply, the electrical voltage across the material will vary in accordance with the infrared energy sensed by the material. Monolithic electronic circuitry connected to the material is used to produce an output signal representative of the infrared energy impinging on the material. By arranging an array of bolometers, together with its output electrical signals, and a processor fed by the output electrical signals can thus be used to provide an electronic image of the source of the infrared energy.
In such application, the infrared sensitive materials are deposited on and the integrated circuitry is fabricated on a substrate or a layer of semiconductor. Most of the infrared sensitive materials of bolometers are suspended on the elevated, air bridging surface member from the substrate by semiconductor micromachining technology to thereby increase its thermal isolation from the substrate. The increased thermal isolation thereby increases the sensitivity of the bolometer to the impinging infrared energy. For example, see U.S. Pat. No. 5,369,280 (Liddiard).
The voltage responsivity of a bolometer, R.sub.v, is defined as: ##EQU1##
where I.sub.b is the bias current, R is the dc resistance, .eta. is the absorptivity, G is the thermal conductance between sensitive element and the substrate, .omega. is the angular modulation frequency of the incident radiation and .tau. is the thermal response time, which is given by C/G. C is the heat capacity (thermal mass) of the sensitive element. .beta. is the temperature coefficient of resistance (TCR) and defined as: ##EQU2##
where T is the temperature. Therefore, for high responsivity, high dR/dT, low G and low .omega. (.omega..tau.&lt;&lt;1) are required. Solid state micromachining techniques can be employed to create an air bridge under the infrared sensitive element to provide low thermal conductance.
The detectivity D* is determined by the ratio of the responsivity R.sub.v to the noise voltage V.sub.n : ##EQU3##
where .DELTA.f is the amplifier frequency bandwidth, .DELTA.V.sub.n is the total noise voltage of the detector, and A is the area of the active region of the detector.
The bolometers can be operated under room temperature, however, these thermal detectors typically have a lower sensitivity and a slower response speed than photon detectors. Accordingly, the improvement in the present invention is directed to a structure which increases the bolometer sensitivity, detectivity and reduces the infrared radiation loss due to reflection.